1. Field of the Invention
This invention relates to optical amplifiers and more particularly to an optical amplifier that uses stimulated polariton scattering for providing optical amplification.
2. Description of the Related Art
Heretofore, the field of optical signal amplifiers has been dominated by Raman amplifiers. For example, U.S. Pat. No. 3,414,354, issued to E. H. Siegler Jr., entitled Raman Spectrometers, is a seminal publication disclosing the use of stimulated Raman scattering, to provide optical amplification. In a later example, U.S. Pat. No. 3,515,897, issued to W. H. Culver, entitled Stimulated Raman Parametric Amplifier, discloses a design for implementing stimulated Raman scattering for amplification. In Physical Review 112, p. 1555-1567 (1958), J. J. Hopfield discusses the effect of exciton absorption on the dielectric response within a crystal. This affects the crystal polariton behavior. The date of the Hopfield paper precedes the first laser demonstration and does not address the stimulated optical response. In Phys Rev 138, p. A1741-A1746 (1965), Y. R. Shen delineates a clear theoretical model for the Raman effect in polar media, an effect closely related to that associated with stimulated polariton scattering. His paper deals strictly with the physical interaction and does not describe applications of the physics. In Physical Review 171 p. 1058-1064 (1968), C. H. Henry et. al. discuss the physical analysis of parametric gain associated with polaritons. They do not address the use of the parametric gain for amplifying a signal. In Applied Physics Letters 15, p. 102-105 (1969), J. M. Yarborough shows how Raman scattering in a polar medium can be used to parametrically generate tunable infrared radiation. In this case, the polar medium is LiNbO3, but there is mention of signal amplification.
Use of Raman scattering for optical signal amplification has limitations in its operation and implementation. Examination of the equations that govern stimulated Raman scattering break down into two terms. The first term is associated with the wave that is being amplified, also known as the Stokes wave. The second term is associated with a material excitation that is a product of the Raman scattering. Consequently, stimulated Raman scattering can be considered as a parametric or coupled generation process in which the optical pump wave generates a Stokes wave (i.e. the amplified input) and a material excitation wave. This material excitation wave is part of the coupled wave physical process which allows the input beam to be amplified. The frequency of the material excitation is set by the material in use. This excitation cannot vary, as it arises from a vibrational state that is both infrared and Raman active. The material excitation frequency imposes a strict frequency relation between the pump and input beams. In many applications, either the input or pump is set by other technical requirements, such as optical power or frequency compatibility, which severely limits the flexibility of the amplifier.